A method for dithering image data is provided. The method is suitable for representing A-bit grayscale values with a B-bit resolution, where A and B are positive integers and A>B. The main steps of the method include calculating 2A target grayscale values with decimals within an interval from 0 to (2B−1)*2A−B, and then representing the target grayscale values with the decimals through 2-dimensional dithering or 3-dimensional dithering.